The present invention relates to a method for hot-dip galvanizing.
Generation of surface defects on hot dip galvanized steel strip caused by dross is one of the most serious problems on the hot-dip galvanized steel strip. Dross is an intermetallic compound such as FeZn7 generated from the reaction between iron and zinc which are eluted from the steel strip in a plating tank holding a zinc-base molten metal, and the dross has spherical equivalent diameters of from 5 to 300 microns. In a stagnant state of the molten metal in the plating tank, the dross deposits on the bottom of the plating tank.
However, with a natural convection of the molten metal, generated from the traveling steel strip, from the rotation of immersed rolls in the tank, or from the dissolved zinc-base ingot that supplies the consumed metal brought out along with the steel strip, the molten metal in the plating tank is agitated. As a result, the dross having less difference in specific gravity from the molten metal cannot deposit on the bottom of tank, or the once-deposited dross is stirred up to adhere the plated steel strip, thus causing the surface defects of the hot-dip galvanized steel strip.
To remove the dross, many proposals have been made. They include a method to sediment the dross by discharging the hot dip zinc bath to outside the plating tank, and a method to filter the hot dip zinc bath.
Nevertheless, those conventionally proposed methods are not brought into practical use. The reason is that these proposed technologies fail in practical application because of many problems in the complex mechanism, the durability, and the operability of commercial facilities, though they are reasonable in theory.
Regarding the methods for sedimentation separation of the dross, which have been proposed, the design emphasizes not to solidify the molten zinc during the transfer to outside the tank, and the design should take into account of the leak accident of molten zinc from the transfer piping. Consequently, the facilities increase the investment cost, which makes the facilities unrealistic ones.
As for the method to filter the dross, there appears a significant difference in the size of intermetallic compounds which can be filtered between the initial period of filtration and the point of degraded filtering performance after clogging the filter unit. As a result, the intermetallic compounds that cause the quality degradation cannot be efficiently and stably removed. Furthermore, on replacing a filter of the filter unit, dismounting and mounting the filter need an additional device that functions in the molten zinc bath, which also needs extra cost as in the case of molten zinc transfer. This also makes the facilities unrealistic ones.
In recent years, there have been proposed methods of direct removal of bottom dross immediately after the generation thereof, which methods stand on different point of view from the conventional methods. Typical examples of the methods are disclosed in JP-A-4-154948, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies xe2x80x9cthe Japanese Patent Laid-Open No.xe2x80x9d), (hereinafter referred to as the xe2x80x9cPrior Art 1xe2x80x9d), JP-A-8-3707, (hereinafter referred to as the xe2x80x9cPrior Art 2xe2x80x9d), and JP-A-7-268587, (hereinafter referred to as the xe2x80x9cPrior Art 3xe2x80x9d).
The Prior Art 1 discloses a method to remove dross in a sedimentation tank installed separately from the plating tank. The characteristics of the method are that the plating tank is designed to decrease the distance between the steel strip and the tank bottom to prevent the sedimentation of the dross, that the transfer of the molten zinc from the plating tank to the sedimentation tank is conducted through a shallow flow passage to let the top dross of the plating tank flow into the sedimentation tank, and that the transfer of the molten zinc from the sedimentation tank to the plating tank is done by a pump.
The Prior Art 2 is characterized in that a flow passage to circulate the molten metal is established by placing a separation plate near an inner wall of the plating tank, that a circulation unit is mounted in the above-described flow passage to circulate the molten metal, that a heating device is mounted at inlet of the above-described flow passage to heat the molten metal to increase the size of dross to enhance the sedimentation of the dross, and that a dross recovery unit is located adjacent to the exit of the above-described flow passage to recover the sedimented dross.
The Prior Art 3 is characterized in that a plating tank having a circular bottom to plating the metal strip and a sedimentation tank to sediment and deposit the bottom dross generated in the plating bath are installed, that a connection hole is opened at near a side wall of the plating tank to let the molten metal for plating freely enter and leave between the plating bath tank and the sedimentation tank, thus the molten metal containing the dross is discharged to the sedimentation tank using the flow accompanied with the traveling steel strip to separate and sediment the bottom dross in the sedimentation tank where the flow rate is slow, and to recycle the molten metal after removed the dross to the plating tank.
According to the Prior Art 1, the suction opening for the molten zinc in the sedimentation tank has to be located at significantly below the bath level, so that the molten zinc containing sedimenting dross is sucked to the opening and transferred to the plating tank. In addition, since the transfer of the molten zinc from the sedimentation tank to the plating tank is conducted by a pump, a large amount of dross is generated in the plating tank which has a discharge opening. That is, the effect of sedimenting and removing the dross is not sufficient, and an additional problem of generation of top dross occurs.
Since the capacity of the sedimentation tank increases and since the problem of solidification and leak of molten zinc during the transfer of molten zinc between the plating and the distant sedimentation tank has not been solved, a problem of increasing investment cost and operation cost arises.
According to the Prior Art 2, the capacity of flow passage should be small as seen in an embodiment described later, so that the effect of sedimenting and removing the large amount of dross generated in the plating tank is not sufficient. Furthermore, the dross sediments and deposits in the flow passage to reduce the capacity of the flow passage, which increases the flow speed of the molten zinc. As a result, necessary sedimenting time cannot be secured, thus degrading the removal efficiency of the dross. In addition, the dross deposited in the narrow flow passage is not easily removed.
According to the Prior Art 3, since the molten zinc is discharged from the plating tank to the sedimentation tank using the flow accompanied with the traveling steel strip, the discharge flow rate cannot be controlled. Therefore, the dross in the plating tank cannot be fully discharged to the sedimentation tank, which raises a problem of accumulation and growth of the dross in the plating tank.
The Prior Art 1 and the Prior Art 3 consider only the flow of molten zinc bath in the cross sectional plane to the direction of traveling steel strip in the plating tank. FIG. 5 and FIG. 6 show schematic drawings of the distribution status of the dross deposited in the plating tank, which are derived from a water model and commercial plant data by the inventors of the present invention. FIG. 5 is a drawing viewed from cross sectional plane to the direction of traveling steel strip in the plating facility. FIG. 6 is a drawing viewed in Axe2x80x94A cross section of FIG. 5. In both drawings, the reference number 2 is a sink roll, and the reference number 8 is the dross.
As seen in FIGS. 5 and 6, the dross 8 deposits at the edge portion of the axial direction of the sink roll 2 and at the front and rear sides of the rotational direction thereof. That is, the flow pattern of the molten zinc between the sink roll and the inner wall surfaces of the plating tank is not a simple one which is represented by one-side cross sectional plane to the direction of traveling steel strip but complex flow in three-dimensional patterns. In many cases, the dross deposits at low flow speed portions of the molten metal, which can be seen in FIGS. 5 and 6. Consequently, it is evident that solely limiting the distance between the steel strip and the tank bottom in the cross sectional plane to the direction of traveling steel strip only changes the place of dross deposition, and the means cannot substantially solve the problem.
Therefore, the above-described Prior Arts fail to prevent the deposition of dross generating during the process of hot hip zinc-base plating and fail to efficiently remove the generated dross.
In the plating tank, the molten metal is consumed by brought out from the plating tank carried by the traveling steel strip. Normally, the make up of the consumed molten metal is done by directly dissolving a solid metal in the plating tank. In addition, it is necessary to control the temperature of the molten metal in the plating tank to a specified level. Ordinary plating tank is provided with an induction-heating device to dissolve the solid metal for plating and to control the temperature of the molten metal to a specified level even when the operating conditions vary.
The inventors of the present invention found that the directly dissolving a solid metal for plating in the plating tank varies the bath temperature in the plating tank, thus significantly enhances the generation and growth of the dross. Furthermore, the inventors found that the high temperature molten metal ejected from the induction-heating device directly contacts the steel strip entering the plating tank, which increases the elution of iron from the steel strip to increase the dross generation. The phenomenon becomes significant in smaller capacity of the plating tank.
To prevent deposition of dross in the plating tank and to efficiently remove the generated dross, it is essential to reduce the volume of generating dross considering the above-described findings. To this point, the above-described Prior Literatures lack the consideration on that point of view.
It is an object of the present invention to provide a method for plating, which method prevents deposition of dross generated during hot-dip galvanizing in a plating tank and which method efficiently removes the generated dross, and to provide an apparatus thereof.
To achieve the object, firstly, the present invention provides a method for hot-dip galvanizing, which comprises the steps of:
dividing a plating vessel which holds a molten metal into a plating tank located at upper portion thereof and a dross removing tank located beneath the plating tank;
conducting hot-dip galvanizing by immersing a steel strip in a molten metal bath in the plating tank;
transferring the molten metal bath from the plating tank to the dross removing tank;
removing dross from the molten metal bath in the dross removing tank; and
recycling the molten metal bath from the dross removing tank to the plating tank through an opening on the plating tank.
The method for hot-dip galvanizing preferably further comprises the step of dissolving a solid phase metal being used for plating in the dross removing tank.
The step of transferring the molten metal bath to the dross removing tank preferably comprises the transferring the molten metal bath from the plating tank to the dross removing tank using a mechanical pump. The step of transferring the molten metal bath to the dross removing tank preferably comprises the transferring the molten metal bath from the plating tank to the dross removing tank by sucking up thereof at bottom center portion of the plating tank.
The step of recycling the molten metal bath to the plating tank preferably comprises the returning the molten metal bath containing a supernatant after removed the dross to the plating tank through an opening of the plating tank. The step of recycling the molten metal bath to the plating tank preferably comprises the returning the molten metal bath from the dross removing tank to the plating tank through a side wall of the plating tank, which side wall is located at exit side of the steel strip and has a height lower than the surface level of the molten metal bath.
The plating tank and the dross removing tank preferably satisfy the relation of W1xe2x89xa610 m3 and W1xe2x89xa6W2, (W1 is the capacity of the plating tank, and W2 is the capacity of the dross removing tank), and the flow rate of molten metal bath being transferred from the plating tank to the dross removing tank is in a range of from 1 to 10 m3/hour.
The step of conducting the hot dip galvanizing is preferably performed in an arrangement that side walls and bottom portion wall are allotted so as the distance between the steel strip and the side walls of the plating tank and between the steel strip and the bottom portion wall of the plating tank is in a range of from 200 to 500 mm.
Secondly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a plating vessel which holds a molten metal;
a plating tank which is located at upper portion of the plating vessel and conducts the hot-dip galvanizing by immersing a steel strip thereinto;
a dross removing tank which is located at lower portion of the plating vessel and which removes dross from the molten metal;
a transfer means which transfers a molten metal bath in the plating tank to the dross removing tank; and
an opening positioned on the plating tank to recycle the molten metal bath from the dross removing tank to the plating tank.
The transfer means is preferably a mechanical pump. The suction opening of the mechanical pump to suck the molten metal is positioned at bottom center portion of the plating tank.
The apparatus for hot-dip galvanizing preferably further comprises a dissolving means to dissolve a solid phase metal being used for plating in the dross removing tank.
The opening is preferably positioned so as the supernatant bath after removed the dross in the dross removing tank to be able to recycle to the plating tank.
The plating tank may have a side wall which is located at exit side of the steel strip and which has a height lower than the surface level of the molten metal bath, and wherein the molten metal bath is recycled from the dross removing tank to the plating tank through the side wall.
The plating tank and dross removing tank preferably satisfy the relation of W1xe2x89xa610 m3 and W1xe2x89xa6W2, (W1 is the capacity of the plating tank, and W2 is the capacity of the dross removing tank), and wherein the mechanical pump is able to transfer the molten metal bath at a flow rate in a range of from 1 to 10 m3/hour.
The plating tank preferably has side walls and bottom portion wall, and these walls are preferably allotted so as the distance between the steel strip and the side wall of the plating tank and between the steel strip and the bottom portion wall of the plating tank is in a range of from 200 to 500 mm. The plating tank preferably has a pipe to fix the bottom portion, through which pipe the draining is conducted.
Thirdly, the present invention provides a method for hot-dip galvanizing, which comprises the steps of:
locating a separation wall inside of a plating tank which holds a molten metal to divide the plating tank into a plating zone where a steel strip is subjected to hot-dip plating, and a dross removing zone where dross in a molten metal bath is removed;
plating the steel strip in the plating zone;
transferring the molten metal bath in the plating zone to the dross removing zone;
removing the dross from the molten metal bath in the dross removing zone; and
recycling a supernatant bath after removed the dross in the dross removing zone by locating a weir on the separation wall.
The step of transferring the molten metal bath to the dross removing zone preferably comprises the transferring the molten metal bath from the plating zone to the dross removing zone using a mechanical pump.
The method for hot-dip galvanizing preferably further comprises a heating device in the dross removing zone to conduct heating control so as the temperature of the molten metal bath in the plating zone to become a predetermined level.
The plating zone preferably has a molten metal bath capacity of W1, and the dross removing zone has a molten metal bath capacity of W2, wherein W1/W2 is in a range of from 0.2 to 5.
Fourthly, the prevent invention provides a method for hot-dip galvanizing, which comprises the steps of:
arranging a separation wall inside of a plating tank which holds a molten metal to divide the plating tank into a plating zone where a steel strip is subjected to hot-dip plating, a first dross removing zone and a second dross removing zone, where a dross in a molten metal bath is removed in the first dross removing zone and the second dross removing zone;
mounting a first mechanical pump to transfer the molten metal bath from the plating zone to the first dross removing zone and locating a weir to recycle the molten metal bath to the plating zone;
mounting a second mechanical pump to transfer the molten metal bath from the plating zone to the second dross removing zone and locating a weir to recycle the molten metal bath to the plating zone;
plating the steel strip in the plating zone;
removing the dross by transferring the molten metal bath from the plating zone to the first dross removing zone using the first mechanical pump; and
discharging the dross deposited in the second dross removing zone to outside the plating tank by stopping the mechanical pump in the second dross removing zone.
Fifthly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a plating tank which holds a molten metal;
a separation wall located in the plating tank to divide the plating tank into a plating zone where a steel strip is subjected to hot-dip plating, and a dross removing zone where dross from a molten metal bath is removed;
a mechanical pump which transfers the molten metal bath from the plating zone to the dross removing zone; and
a weir located to the separation wall to transfer a supernatant bath of the molten metal bath after removed the dross in the dross removing zone to the plating zone.
The apparatus for hot-dip galvanizing preferably further comprises a heating device which is located in the dross removing zone and which controls the temperature of molten metal bath by heating thereof.
The plating zone preferably has a molten metal bath capacity of W1, and the dross removing zone has a molten metal bath capacity of W2, wherein W1/W2 is in a range of from 0.2 to 5.
Sixthly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a plating tank which holds a molten metal;
a separation wall located in the plating tank to divide the plating tank into a plating zone where a steel strip is subjected to hot-dip plating, and a dross removing zone where dross in the molten metal bath is removed;
the dross removing zone comprising a first dross removing zone and a second dross removing zone;
a first mechanical pump which transfers the molten metal bath from the plating zone to the first dross removing zone;
a second mechanical pump which transfers the molten metal bath from the plating zone to the second dross removing zone;
a first weir located to the separation wall to transfer a supernatant bath of the molten metal bath after removed the dross in the first dross removing zone to the plating zone; and
a second weir located to the separation plate to transfer a supernatant bath of the molten metal bath after removed the dross in the second dross removing zone to the plating zone.
Seventhly, the present invention provides a method for hot-dip galvanizing, which comprises the steps of:
arranging a separation wall inside of a plating tank which holds a molten metal to divide the plating tank into a plating zone where a steel strip is subjected to hot-dip plating, and a dross removing zone where dross in a molten metal bath is removed;
continuously plating the steel strip in the plating zone using a sink roll;
transferring the molten metal bath above the sink roll in the plating zone to the dross removing zone using a mechanical pump;
removing the dross from the molten metal bath in the dross removing zone; and
recycling a supernatant bath after removed the dross in the dross removing zone to the plating zone via a weir located on the separation wall.
The method for hot-dip galvanizing preferably further comprises a step of locating a heating device in the dross removing zone to conduct heating control so as the temperature of the molten metal bath in the plating zone to become a predetermined level.
The plating zone preferably has a molten metal bath capacity of W1, and the dross removing zone has a molten metal bath capacity of W2, wherein W1/W2 is in a range of from 0.2 to 5.
Eighthly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a plating tank which holds a molten metal;
a sink roll which makes a steel strip immerse in and travel through the molten metal;
a separation wall located in the plating tank to divide the plating tank into a plating zone where the steel strip is subjected to hot-dip plating, and a dross removing zone where dross in the molten metal bath is removed;
a mechanical pump which transfers the molten metal bath above a sink roll in the plating zone to the dross removing zone; and
a weir located on the separation wall to transfer a supernatant bath of the molten metal bath after removed the dross in the dross removing zone to the plating zone.
The apparatus for hot-dip galvanizing preferably further comprises a heating device which is located in the dross removing zone and which controls the temperature of the molten metal bath by heating thereof.
The plating zone preferably has a molten metal bath capacity of W1, and the dross removing zone has a molten metal bath capacity of W2, wherein W1/W2 is in a range of from 0.2 to 5.
Ninthly, the present invention provides a method for hot-dip galvanizing, which comprises the steps of:
locating a sink roll which guides a steel strip traveled through a snout into a plating vessel which holds a molten metal;
separating the plating vessel into a plating zone and a dross removing zone by locating a plating tank so as to cover the sink roll, and by locating a shielding member to shield a gap formed between a lower portion of the snout beneath the steel strip and an upper portion of the plating tank;
conducting hot-dip galvanizing by immersing the steel strip in the plating zone;
removing dross from a molten metal bath in the plating zone by discharging the molten metal bath from the plating zone to the dross removing zone using a mechanical pump; and
recycling the molten metal bath from the dross removing zone to the plating zone.
The plating tank is preferably located so as the upper end of the plating tank to become higher than the level of a rotary shaft of the sink roll.
Tenthly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a snout through which a steel strip travels;
a plating vessel which holds a molten metal, which plating vessel has a sink roll to guide the steel strip traveled through the snout;
a plating zone to conduct hot-dip galvanizing by immersing the steel strip thereinto and a dross removing zone to remove dross from a molten metal bath, which zones are formed by locating a shielding member to shield a gap formed between a lower portion of the snout beneath the steel strip and an upper portion of a side wall of the plating tank; and
a mechanical pump to discharge the molten metal bath from the plating zone to the dross removing zone and also to recycle the molten metal bath from the dross removing zone to the plating zone.
The plating tank is preferably located so as the upper end of the plating tank to become higher than the level of a rotary shaft of the sink roll.
Eleventhly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a plating bath tank which holds a hot-dip galvanizing bath containing aluminum at contents of 0.05 wt. % or more;
a snout through which a steel strip immersed in the plating bath tank travels;
a plating tank which conducts plating and a dross removing tank which separates dross by sedimenting the dross, both of which tanks are formed by locating a separation wall in the plating bath tank;
a snout cleaning device to connect the plating tank and the dross removing tank at directly below the snout and at a part of exit of the steel strip so as a connecting passage to have 0.1 meter or more hydraulic diameter defined by a formula given below and so as the bath levels of both tanks to become equal to each other, to suck the plating bath in the snout by a pump from both longitudinal edges of the snout to discharge the sucked bath to a portion where no steel strip travels, thus cleaning the plating bath surface in the snout, and to circulate the plating bath between the plating tank and the dross removing tank; wherein the hydraulic diameter is defined as
Hydraulic diameter={(Cross sectional area of flow passage)/(Wet length of flow passage)}xc3x974.
The capacity of the plating tank is preferably not more than 10 m3, and the capacity of the dross removing tank is not more than 10 m3.
Twelfthly, the present invention provides a method for hot-dip galvanizing, which comprises the steps of:
locating a separation wall in a plating bath tank which holds a hot-dip galvanizing bath containing aluminum in an amount of 0.05 wt. % or more to divide the plating bath tank into a plating tank which conducts plating and a dross removing tank which dissolves an ingot and which separates dross by sedimenting thereof;
connecting the plating tank and the dross removing tank at directly below the snout and at a part of exit of the steel strip so as a connecting passage to have 0.1 meter or more hydraulic diameter defined by a formula given below and so as the bath levels of both tanks to become equal to each other, and sucking the plating bath in the snout by a pump from both longitudinal edges of the snout to discharge the sucked bath to a portion where no steel strip travels, thus cleaning the plating bath surface in the snout and circulating the plating bath between the plating tank and the dross removing tank, wherein the hydraulic diameter is defined as
Hydraulic diameter={(Cross sectional area of flow passage)/(Wet length of flow passage)}xc3x974.
The capacity of the plating tank is preferably 10 m3 or less, the capacity of the dross removing tank is 10 m3 or more, the circulation flow rate of the plating bath between the plating tank and the dross removing tank is between 0.5 and 5 m3/hour.
Thirteenthly, the present invention provides an apparatus for hot-dip galvanizing, which comprises:
a molten zinc tank which holds a molten zinc and has a heating means for heating the molten zinc;
a sink roll which is immersed in the molten zinc in the molten zinc tank and around which a steel strip is wound;
a vessel which holds the sink roll therein and comprises side panels and a bottom panel, while opening the upper end thereof;
whereby hot-dip galvanizing is performed to a continuously fed steel plate in the molten zinc tank.
The heating means of the molten zinc tank preferably conducts coreless induction heating.
The vessel preferably keeps gaps of from 200 to 500 mm between the vessel walls and the steel strip traveling through the vessel, the sink roll, and a jig to fix the sink roll.
The apparatus for hot-dip galvanizing further comprises a cover which substantially covers the lower surface of the steel strip being immersed in the molten zinc in the molten zinc tank until the steel strip reaches the vessel.
The vessel preferably has a curved face at joints of the side plates and the bottom plate.
The vessel preferably has a discharge opening at the bottom thereof to discharge the molten zinc, through which discharge opening the molten zinc is forcefully discharged into the molten zinc tank.
Fourteenthly, the present invention provides a method for hot-dip galvanizing, which comprises the steps of:
dividing a plating vessel which holds a molten metal into a dross removing tank and a plating tank which is located in the dross removing tank;
conducting hot-dip galvanizing by immersing a steel strip in a molten metal bath in the plating tank;
transferring the molten metal bath from the plating tank to the dross removing tank using a mechanical pump and using a flow accompanied with the traveling steel strip appeared at a first opening;
removing a dross from the molten metal bath in the dross removing tank; and
recycling the molten metal bath from the dross removing tank to the plating tank via a second opening located on the plating tank.
The plating tank preferably keeps gaps of from 200 to 500 mm between the walls of plating tank and the steel strip, and between the walls of plating tank and the sink roll in the bath, and wherein the plating tank and the dross removing tank preferably satisfy the relation of W1xe2x89xa610 m3 and W1xe2x89xa6W2, (W1 is the capacity of the plating tank, and W2 is the capacity of the dross removing tank), and the flow rate of molten metal bath being transferred from the plating tank to the dross removing tank is preferably in a range of from 1 to 10 m3/h.
Fifteenthly, the present invention is to provide an apparatus for hot-dip galvanizing, which comprises:
a plating vessel which holds a molten metal, wherein the plating vessel comprises a dross removing tank which removes dross from the molten metal, and a plating tank which is located in the dross removing tank and which conducts hot-dip galvanizing to a steel strip;
a transfer means which transfers a molten metal bath from the plating tank to the dross removing tank;
a first opening which is located at the plating tank and functions to transfer the molten metal bath from the plating tank to the dross removing tank using a flow accompanied with the traveling steel strip; and
a second opening which is located at the-plating tank and which functions to recycle the molten metal bath from the dross removing tank to the plating tank.
The plating tank preferably keeps gaps of from 200 to 500 mm between the walls of plating tank and the steel strip, and between the walls of plating tank and the sink roll in the bath, and wherein the plating tank and the dross removing tank preferably satisfy the relation of W1xe2x89xa610 m3 and W1xe2x89xa6W2, (W1 is the capacity of the plating tank, and W2 is the capacity of the dross removing tank).